This invention relates to improvements in the field of scanning electron microscopes. More particularly, in a first aspect this invention relates to apparatus and method for coating non-conductive specimens prior to examination in a scanning electron microscope, hereinafter designated an SEM. In another aspect, this invention relates to method and apparatus for facilitating the handling of both conductive and non-conductive specimens in an SEM.
SEM systems are known in which a specimen is placed on a mounting platform in an enclosed main chamber and investigated by scanning with an electron beam. Typically, fine vernier adjustments are mechanically linked to the platform so that the specimen may be precisely positioned with respect to the beam. Also, in a typical SEM system a vacuum pump is ordinarily provided for evacuating the main chamber after the specimen has been installed in order to provide a more favorable environment for the ensuing scanning. Thus, in a typical operation of known SEM systems, the specimen is first placed inside the main chamber, the chamber being entered through an access door having a vacuum seal, and the main chamber is pumped down to the desired low pressure by means of the vacuum pump. Since the volume of the main chamber is substantial, the scanning investigation cannot therefore proceed immediately after installation of the specimen in the main chamber, but must be delayed until such time as the vacuum pump is able to evacuate the chamber to the requisite subatmospheric pressure. This problem is compounded by the fact that the main chamber must be completely backfilled and then re-evacuated each time a previously installed specimen is removed after scanning and a new specimen is installed therein.
It is also frequently desirable when scanning certain types of specimens, particularly biological specimens, to perform the scanning process with the specimen held at an extremely low temperature of cryogenic order. In the past, this has been accomplished by first freezing the specimen with a low temperature cooling fluid in a tank located outside of the main chamber, and then inserting the frozen specimen into the main chamber onto the specimen platform. During the time that the frozen specimen is being transferred from the freezing solution to the platform, however, any contact, however momentary, with the ambient has been found to cause frost formation on the surface of the specimen. This surface layer of frost introduces inaccuracies in the scanning process and leads to inaccurate identification of the substances composing the specimen. While efforts have been made to shield a frozen specimen from the ambient until installation in the main chamber such efforts have not proven altogether effective in eliminating frost formation, particularly since the main chamber must of necessity be at ambient pressure during specimen installation.
In addition, once installed, the specimen must be maintained at the requisite low temperature in order to prevent un-freezing during the scanning process. Efforts to provide a workable cooling system to maintain the specimen in the frozen state have led to complicated, cumbersome and expensive arrangements which have not been found to be satisfactory to date.
Since the scanning process ordinarily requires a specimen that is electrically conductive, it is also frequently necessary to coat non-conductive specimens with some substance, such as gold, having good electrical conductivity. In known systems, the specimen is typically coated in a vacuum deposition coating device located outside and remote from the main chamber, and subsequently transferred thereto. In addition to the inconvenience of transferring the specimen to the site of the vacuum deposition apparatus for coating, and returning the coated specimen to the site of the SEM, the same severe problems of frost formation on a frozen coated specimen are encountered.
Efforts at providing SEM equipment which avoids the above-noted disadvantages have not met with wide success to date.